This study will comprehensively assess the neuropsychological performance of hyperthyroid children and adults both during thyrotoxicosis and following establishment of euthyroid status. The hypotheses are: 1) Thyrotoxic patients will show significant neuropsychological deficits. 2) Neuropsychological deficits will improve significantly or normalize following achievement of euthyroid status. This project is just beginning accrual of subjects. The GCRC psychometrist is the primary investigator on this project.